Is it just tradition or is there an actual reason?
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Note that $(n-1)! = \color{blue}{(n-1)}(n-2)\cdots (2)(1) = \color{blue}{(n-1)}(n-2)!$ by plugging in $(n-2)! = (n-2)\cdots (2)(1)$ into the expression for $(n-1)!$.
So $$\frac{(n-1)!}{(n-2)!} = \frac{(n-1)(n-2)!}{(n-2)!} = n-1$$
you already know that $$n!=\left( n-1 \right) !n\\ \left( n-1 \right) !=\left( n-2 \right)! \left( n-1 \right) \\ \left( n-2 \right) !=\left( n-3 \right) !\left( n-2 \right) \\ ......$$ so
$$\frac { \left( n-1 \right) ! }{ \left( n-2 \right) ! } =\frac { \left( n-2 \right) !\left( n-1 \right) }{ \left( n-2 \right) ! } =n-1\\ $$
I feel like you should be able to, but when you plug zero in for n, you get different answers.
Can anyone tell me how to simplify both of these expressions? Really having a hard time with this as i can't fully understand any teacher who tries to explain it.
$$\require{cancel}n!=\overbrace{1\times2\times3\times\dots\times(n-1)\times n}^n$$
$$\frac{n!}n=\frac{\overbrace{\color{green}{1\times2\times3\times\dots\times(n-1)}\times\color{red}{\cancel n}}^n}{\color{red}{\cancel n}}$$
$$(n-1)!=\overbrace{\color{green}{1\times2\times3\times\dots\times(n-1)}}^{n-1}$$
We also have:
$$\frac{n!}{p!}=\frac{\overbrace{\color{red}{\cancel{1\times2\times3\times\dots\times p}}\times\color{green}{(p+1)\times\dots\times(n-1)\times n}}^n}{\color{red}{\cancel{1\times2\times3\times\dots\times p}}}$$
$$=\color{green}{(p+1)\times\dots\times(n-1)\times n}$$
The factorial can resursively defined as $0! = 1$ and $$ n! = n\cdot(n-1)!$$ for $n\geq 1$.
An example is $4! = 4\cdot 3! = ... = 4\cdot 3\cdot 2\cdot 1.$ Hence by isolating the $(n-1)!$ term we get $$(n-1)! = \frac{n!}{n}.$$